1. Field of the Invention
This invention relates to hot melt adhesives based on ethylene copolymers. In particular, the invention relates to hot melt adhesives especially comprising ethylene alpha-olefin copolymers wherein the alpha-olefin can have from 3-20 carbon atoms such as ethylene/butene-1, ethylene/hexene-1, ethylene/octene-1, and ethylene/4-methylpentene-1 copolymers. These ethylene copolymers with prescribed range of comonomer levels can be prepared by polymerization of the suitable olefins in the presence of supported or unsupported cyclopentadienyl transition metal compounds and organo aluminum reagent catalyst systems.
2. Background of the Invention
Hot melt adhesives are mixtures of polymer and adjuvants, usually tackifying resin, wax, or low T.sub.g materials such as oils and low molecular weight polymers. The polymer contributes cohesive strength to the final formulation. It is well known that highly crystalline polymers show very poor adhesive performance at temperatures below room temperature due to brittleness. Hence, somewhat elastic polymers are most useful in hot melt adhesives. In addition, the presence of comonomer improves adhesion of the final hot melt adhesive formulation to metal and polar substrates such as aluminum and glass.
Copolymers of ethylene and a mono-olefinically unsaturated polar comonomer such as vinyl acetate, methyl acrylate, acrylic acid and the like, have been reported as useful polymers to formulate into hot melt adhesives, however, these hot melt adhesives are generally lacking in elevated temperature performance. These hot melt adhesives, although useful with polar substrates, are less useful with non-polar substrates.
Hot melt adhesives prepared with EVA (ethylene/vinyl acetate copolymer) containing high levels of vinyl acetate are generally ineffective in laminating non-polar plastic surfaces to the same plastic surfaces or to other surfaces. For example, polypropylene is widely used for product assembly in automobile industry due to low cost and solvent resistance. The polypropylene parts require a hot melt adhesive that will bond the polypropylene as well as to parts made of different polymers or to other substrates such as steel, aluminum, glass, etc. Each of the different polymers or substrates may have different bonding requirements, such as polarity, etc. Besides excellent adhesion, another highly desirable requirement of the hot melt composition is its elevated temperature properties. Besides product assembly applications, polypropylene is widely used in film and in cast sheet form for packaging articles and for packaging food and such packaging requires closure which can resist mechanical breakage.
U.S. Pat. No. 4,072,735 discloses the preparation of a hot melt pressure sensitive adhesive which comprises ethylene-propylene rubber, tackifying resin, polybutene, and, optionally, a crystalline polypropylene. The addition of the crystalline polypropylene is most likely required in order to obtain a useful adhesive at elevated temperatures.
U.S. Pat. No. 4,568,713 discloses a hot melt adhesive system comprising butene-1/ethylene copolymer (containing 5.5-10.0 weight percent ethylene), an aliphatic/non-polar tackifier, an antioxidizing agent and, optionally, microcrystalline wax. The patent discloses utility for long open time and the need for good cold metal bonding to steel and aluminum.
JP-62-129303 discloses the preparation of ethylene/alpha-olefin copolymers using metallocene-alumoxane catalysts. The products obtained are olefin copolymers having narrow molecular weight distributions and relatively low softening points. These copolymers are waxes characterized by only slight surface stickiness or slight adhesiveness, and are therefore used as pigment dispersing agents and toners.
JP-61-236B04 discloses preparing narrow molecular weight distribution ethylene/alpha-olefin copolymers with metallocene-alumoxane catalysts. However, these ethylene/alpha-olefin copolymer products are also waxes that are used as pigment dispersants and resin processing aids. No adhesive characteristics are taught for these products.
JP-62-121709 pertains to the copolymerization of ethylene with an alpha-olefin in the presence of a catalyst comprising a zirconium hydride compound such as a bis(cyclopentadienyl) zirconium monochlorohydride and an alumoxane to obtain a copolymer which is narrow in both molecular weight distribution and composition distribution. However, while the copolymer is excellent in transparency, impact resistance and heat sealability, it is also characterized by surface non-stickiness.
There is an apparent need in the art to obtain hot melt adhesives having improved elevated temperature properties, and better low temperature flexibility.
In copending application, Ser. No. 07/691,159, filed Apr. 24, 1991, which is continuation in part of U.S. Ser. No. 406,935 filed Sep. 13, 1989, it was disclosed that hot melt adhesives based on ethylene copolymers provide good adhesive performance on plastic surfaces, in particular surfaces comprising polypropylene and polyethylene. U.S. Ser. No. 691,159 also disclosed a hot melt adhesive which does not need the addition of an elastomer to modify final performance such as increased high temperature resistance and improved adhesion on non-polar surfaces.
U.S. Ser. No. 691,159 discloses that ethylene/alpha-olefin copolymers having a certain molecular weight range and a certain comonomer level range can be utilized in formulating an adhesive formulation absent an elastomer when the copolymers are prepared in the presence of a catalyst system comprising a Group IVB transition metal-cyclopentadienyl containing catalyst such as, for example, either supported or unsupported cyclopentadienyl transition metal compounds and organo aluminum reagents catalyst systems. The hot melt adhesive formulations of U.S. Ser. No. 691,159 comprise the copolymers, tackifier(s), and optionally, wax or oil. In hot melt adhesive applications, the weight average molecular weight (M.sub.w) values of the base copolymer will range from about 20,000 to about 100,000, preferably from 30,000 to 80,000, and the comonomer level will range from about 3 mole percent to about 17 mole percent, preferably from 5 mole percent to 11 mole percent.